The detection of defects on patterned wafers is a critical problem in the semiconductor industry. Defects in one or more of the photolithographic patterns can produce nonfunctioning or substandard devices. It is important to identify the type and characteristics of any defects in the integrated circuits at various processing stages so that the cause of the defects can be corrected before it adversely affects the yield.
In the prior art, there are many ways to locate defects on the surface of a semiconductor wafer. One typical defect location method is image analysis of dark field images. There are presently several different methods of illuminating the surface of the wafer so that defects can be found. Generally, these illumination methods fall into one of two classes: the in-lens, on-axis illumination technique and the off-lens, off-axis illumination technique.
The terms "in-lens" and "off-lens" refer to whether an incident beam of light produced by an inspection system travels through or does not travel through, respectively, an inspection (e.g., Fourier transform) lens before striking the surface of a semiconductor wafer. Similarly, the terms "on-axis" and "off-axis" refer to whether an illumination beam, a Fourier transform lens, and the semiconductor wafer under inspection are all positioned or are not positioned, respectively, along an axis.
One such prior art reference is U.S. Pat. No. Re. 33,956 entitled INSPECTION SYSTEM FOR ARRAY OF MICROCIRCUITS DIES HAVING REDUNDANT CIRCUIT PATTERNS, which is hereby incorporated by reference for its teachings in optics and the inspection of semiconductor wafers, describes an in-lens, on-axis illumination technique. This system uses an illumination beam, a Fourier transform lens, an inverse Fourier transform lens, and a semiconductor wafer which are all positioned along an optic axis.
The system forms a dark field image of an area on the wafer at a distant image plane. In this system, spatial frequencies corresponding to the repetitive patterns are selectively attenuated in the dark field image by inserting a spatial filter at a Fourier transform plane between the two lenses. The resulting image accentuates irregularities on the surface of the integrated circuit, such as may result from defects in the pattern.
In this system, the illuminating beam illuminates the wafer through the Fourier transform lens. Thus, light scattered and reflected by the lens adds background noise to the image. The light scattering problem can be reduced by surface cleaning of the lens, coating of the lens, and proper selection of the lens materials. In addition, specular reflections from the Fourier transform lens also add background noise to the image. This background noise can be reduced significantly by using an anti-reflection coating on the lens and by using judiciously sized and placed stops along the optical axis. Nonetheless, background noise resulting from specular reflections will reach the detection device and does reduce the sensitivity of the detection system.
This system collects spatial frequencies of the defects in the wafer which are symmetrically disposed about an origin in the Fourier transform plane that includes two orthogonal axes and four separate quadrants. Because the defects in the wafer can usually be considered planer, the spatial frequency spectrum associated with these defects is symmetrical along both axes in the Fourier transform plane. Thus, this system collects symmetrical spatial frequency components which provide redundant defect information and limits the highest spatial frequency this system can collect. Accordingly, this inspection system may be less sensitive to some relatively small sized defects in the wafer which have relatively more high frequency components.
An example of an off-lens, off-axis illumination scheme is given in U.S. Pat. No. 5,177,559 entitled DARK FIELD IMAGING DEFECT INSPECTION SYSTEM FOR REPETITIVE PATTERN INTEGRATED CIRCUITS, which is hereby incorporated by reference for its teachings in optics and the inspection of semiconductor wafers.
This system forms a dark field image of an area on the wafer. In this system, a semiconductor wafer is illuminated with a light beam at a grazing angle of incidence of between 8 degrees and a predetermined maximum angle with respect to the wafer surface. Light scattered at angles within a given range about the normal to the wafer surface is collected by a lens system which spatially filters the collected light so as to attenuate spatial frequency components corresponding to repetitive patterns. The remaining light is focused and forms an image which accentuates irregularities on the surface of the wafer such as may result from defects in the pattern.
In this system, most of the reflected light reflects away from, rather than toward, the lens system. Only a very small percentage of the total reflected light scatters at angles normal to the wafer so that the lens system can collect this scattered light. Thus, to collect a sufficient amount of scattered light, the lens system is usually located very close to the surface of the wafer. Typically, in applications requiring detection of submicron defects, the space between the lens system and the wafer surface is such that introducing the illumination beam is difficult and limits the lens design forms available for the Fourier transform lens (e.g., lens size, numerical aperture, and working distance).
This system collects spatial frequencies of the defects which are in the higher range. The collected spatial frequencies in the higher range are collected from within one of the four quadrants within the Fourier transform plane; therefore, this inspection system fails to use the maximum collection capability of the lens system. It also means that this inspection system can be less sensitive to some relatively large sized defects that have relatively low frequency components.